Blue or red ΔνXH complexation shift in X-H⋯CO2 hydrogen-bonded complexes?

► The blue-shifting HB is characteristic for donors with a small positive charge on the H atom. ► These proton donors exhibit long and weak X-H bonds. ► As a rule, they form weak complexes with the CO2 molecule. ► The red-shifting HB occurs for molecules with greater positive charge on the H atom.